1. Technical Field
Embodiments of the present invention relate to a sheet conveyor for feeding and conveying a sheet for image forming, and an image forming apparatus such as a printer, facsimile machine, copier, and so forth including the sheet conveyor.
2. Related Art
As one type of a sheet conveyor that loads a stack of sheets of originals or recording media sheets and feeds the stack of sheets one by one toward a subsequent stage, an electrostatic sheet feeder that attracts and separates a sheet electrostatically has been proposed.
One example of such a sheet conveyor includes an attraction/separation unit including a dielectric attraction belt stretched around two rollers, a charger to charge the attraction belt with an AC charge, and a holder to hold the attraction belt and the charger and which rotatably supports the two rollers. The holder is fixed to a rotation shaft provided upstream from the two rollers in a sheet conveyance direction. Further, a mechanism is provided to swing the attraction/separation unit about the rotation shaft so that the attraction belt moves reciprocally between a sheet attraction position and a sheet conveyance position. The sheet attraction position is where the attraction belt contacts an uppermost sheet of a sheet stack that is placed on a bottom plate of a sheet tray to attract the uppermost sheet. The sheet conveyance position is where the attraction belt separates from the sheet stack and conveys the uppermost sheet attracted thereto to a later stage.
Before conveyance of the sheet, the attraction belt held by the holder via the two rollers remains separated from the sheet stack. When separating the uppermost sheet from the sheet stack for transfer, one of the two rollers functions as a driving roller that is rotated by a drive source and that rotates the attraction belt for applying an alternating charge to the attraction belt. After being charged, the attraction belt stops and the mechanism moves the attraction/separation unit toward the sheet stack to the sheet attraction position. In consequence, the attraction belt contacts the uppermost sheet of the sheet stack for attraction.
When the uppermost sheet of the sheet stack contacts the attraction belt, the mechanism moves the attraction/separation unit in a direction to separate from the sheet stack. Consequently, the attraction belt lifts the uppermost sheet attracted thereto, resulting in separation of the uppermost sheet from the rest of the sheet stack. Upon the attraction belt reaching the sheet conveyance position, the driving roller rotates the attraction belt to convey the uppermost sheet onward to a later stage.
The electrostatic attraction force generated between the attraction belt and the uppermost sheet increases as the attraction belt approaches the uppermost sheet. For best results, it is preferable that the attraction belt be pressed against the uppermost sheet with a constant force sufficient to uniformly contact the attraction belt against the uppermost sheet. Accordingly, the sheet conveyor includes a planar pressing member disposed inside the loop formed by the attraction belt and a spring to bias the pressing member to contact against the inner surface of the attraction belt.
However, friction generated between the attraction belt and the pressing member imposes a load on the attraction belt that may cause the attraction belt to slip on the driving roller and result in a sheet conveyance failure.